1. Field of the Invention
The method and device according to the invention relate to optical wave surface analysis, that is to say the measurement of shape parameters of the surface of a wavefront, or wave surface, for ultraviolet, visible or infrared wavelengths. The wave surface is the cartography of the phase of the wavefront.
The shape parameters can, for example, be curvature, tilt or standard geometrical aberrations: spherical aberration, astigmatism, coma, etc. . . More generally, the phase "PHgr" of the incident wavefront in an optical system can be broken down on a basis of orthogonal polynomials Pi over the format formed by the pupil of the optical system:
"PHgr"=xcexa3ai Pi
The measurement of shape parameters then involves the determination of the coefficients ai. Hereafter, the support constituted by the pupil of the optical system will be referred to as the xe2x80x9cwavefront supportxe2x80x9d.
2. Description of the Relate Art
Different types of device are known for carrying out wave surface analysis. Interferometric devices for example determine the wave surface on the basis of the interference pattern between a reference wavefront and the wavefront to be studied. Detection of the interference pattern is carried out directly using a two-dimensional array of detectors (typically 512xc3x97512 elementary detectors or xe2x80x9cpixelsxe2x80x9d) which allows a high spatial resolution and a very fine analysis of the wave surface. However, this method uses a large amount of signal processing because of the quantity of data to be managed and because of this requires a long measurement time which is difficult to make compatible with real time measurements. Interferometric devices are furthermore complex to use and have operational constraints due to the physical mechanism on which they are based: small measurement dynamic, operation using monochromatic light, difficulty in analysing the wavefront from dioptric systems, impossibility of analysing a wavefront coming directly from a source such as a laser diode for example.
Wavefront analysers of the Shack-Hartmann type do not have these constraints. Their use is standard for example in adaptive optics for correcting in real time phase disturbances introduced by turbulence in the propagation of optical beams (ONERA French patent FR A-2665955). Other documents describe devices based on the Shack-Hartmann array technology (see for example the patents U.S. Pat. No. 4,737,621 and U.S. Pat. No. 5,233,174). The principle of the Shack-Hartmann analyser consists in sampling the optical wavefront using a two-dimensional array of optical systems, conventionally spherical microlenses, the array being conjugate with the entrance pupil of the optical system. Each microlens defines a sub-pupil and gives an image of the object. The local slopes of the wave are determined by the relative displacement of the focal spot with respect to a reference position corresponding to a plane wave, coming from an object at infinity. The wave surface is reconstructed by integration of the local slope measurements. The mathematical principle of this reconstruction is described for example in the article by SOUTHWELL W. H. (xe2x80x9cWave-front estimation from wave-front slope measurementsxe2x80x9d, JOSA Vol. 70 nxc2x08 pp 988-1006) for estimating the phase of a circular support wavefront on the basis of local slope measurements. In order to measure the displacements, an array of CCD type detectors is placed at the focus of the microlenses. The production of this type of analyser leads to a simple, compact and totally achromatic optical instrument.
However, a major disadvantage of devices of the Shack-Hartmann type is due to their poor spatial resolution. In fact, in order to minimise the measurement error on the average slope of the wavefront over a sub-pupil, proportional to the displacement of the image spot at the focus of the corresponding microlens, it is necessary that the measurement of this displacement be carried out using a sufficient number of pixels of the array of detectors. Conventionally, about 15xc3x9715 pixels are associated with one sub-pupil. Thus, the number of sub-pupils is limited by the size of the array of detectors. The error in the reconstruction of the wavefront due to the spatial sampling of the latter becomes greater as the number of sub-pupils becomes smaller; on the one hand because of the filtering effect of high spatial frequencies of the phase of the wavefront, and on the other hand because of the poor geometrical cover of the optical wavefront with the array of sub-pupils. The patent U.S. Pat. No. 5,493,391 proposes a one-dimensional wavefront distortion sensor for studying fluid mechanical phenomena with a very good spatial and temporal resolution, but it does not make it possible to determine the shape parameters of the whole surface of the wavefront.
Other types of wave surface analysers are known; for example, curvature analysers or bilateral or trilateral shift analysers. The physical principles of the means of detection used in all these devices differ, but all use a wave surface analysis process based on an overall treatment of the latter: the detection means comprise arrays of two-dimensional detectors making it possible to detect the whole of the wave surface to be analysed. This method has two major disadvantages:
Even if a specific application requires only the measurement of certain shape parameters, the amount of processing of this data will not be less nor will the measurement time be shorter.
This method is linked to the use of arrays of detectors and on the technology appropriate for it, giving rise in particular to limits in the size of the useful analysis zone. This explains why the majority of the known wave surface analysers require the use of shaping optics in order to adapt the diameter of the wavefront to the field of the analysis device. Furthermore, this gives rise to a limitation of the spatial resolution and of the measurement accuracy in particular in devices of the Shack-Hartmann type.
The present invention departs from the conventional method of wave surface analysis based on the acquisition of the whole of the wavefront and proposes a method for analysing the surface of the wavefront based on a line-by-line acquisition of the wavefront and a method of measuring shape parameters which is adapted to this.
More precisely, the invention relates to a method for measuring a given number of shape parameters of the surface of an optical wavefront of given support, the shape parameters being the coefficients of a break-down of the phase of the wavefront on a base of orthogonal polynomials over said format, characterised in that it comprises:
the acquisition of at least two separate lines of the wavefront comprising, for each wave line, the optical detection of said line delivering an electrical signal characterising it, and the processing of said signal making it possible to determine a set of parameters proportional to the values of the phase of the wavefront, or of an nth derivative of said phase, measured on said line,
the calculation, from the sets corresponding to the said wave lines, of said shape parameters.
Advantageously, the calculation step comprises a step for reconstructing each wave line consisting, for example, in expressing the phase of each wave line on a base of orthogonal polynomials, then a step of reconstruction of the wave surface from the reconstructed lines.
The invention also relates to a device for measuring shape parameters of the surface of a wavefront which uses the method according to the invention.
The device according to the invention advantageously comprises, for the detection of a wave line, a detection module comprising, in particular, a set of optical systems disposed linearly and a strip of detectors. Strips of detectors are easier to produce than the equivalent two-dimensional detector arrays and, for the same cost, thus have a larger size. Consequently, the spatial resolution can be improved in comparison with that obtained by array wavefront analysers of the Shack-Hartmann type. The device according to the invention furthermore makes it possible, in most cases, to dispense with shaping optics which are generally necessary in the wavefront analysers of the prior art in order to adapt the diameter of the wavefront to the field of the analysis system.
The device according to the invention also has the advantage of being modular and adaptable. In fact, the number of wavefront lines to be acquired depends on the number and nature of the shape parameters required to be measured. Thus the calculating capacity and the measuring time are adapted to each application.